Promotion of flowering in fruit trees

ABSTRACT

The flowering of broad-leaved trees, particularly fruit trees, is promoted by applying as a foliar treatment, a selected rapidly metabolisable gibberellin chosen from among gibberllin A 4 , C-3 epi-gibberellin A 4 , gibberellin A 1 , and salts and esters thereof. Use of the method of invention can promote return flowering and prevent biennial bearing.

This specification is a continuation of application Ser. No. 824,875,filed Jan. 31, 1986 now abandoned.

This invention relates to a method of promoting flowering ofbroad-leaved trees, particularly so as to overcome problems of irregularcropping, and most notably the horticultural problem commonly referredto as biennial bearing and to a composition for use in such method.

Biennial bearing is a considerable problem with a number of woodyangiosperms including apple, apricot, pistachio, pecan, coffee andseveral species of citrus. It can result in a glut of fruit in one yearand very low production the next. The low crop in the "off" year isdirectly related to the failure by the plant to produce an adequatenumber of flowers on spur shoots, or on branches bearing fruit theprevious season. If the tendency toward biennial bearing is not toosevere, the problem can be controlled by the use of growth retardants,ethylene generating chemicals, girdling, and the aggressive use ofmanual or chemical `thinning` procedures to reduce the crop in the `on`year. However, severe biennial bearing cannot be corrected by suchprocedures.

It has now been found that field applications of carefully selectedgibberellins can promote `return flowering`.

This discovery is particularly surprising because some otherhorticultural treatments involving gibberellins (i.e. treatments topromote fruit set or improve aspects of fruit quality) often antagonizethe next year's flowering.

Thus according to the present invention there is provided a method ofpromoting flowering of a woody angiosperm that is prone to severebiennality which comprises applying an effective amount of a rapidlymetabolizable gibberellin. It is preferred to apply a gibberellin nativeto the species (or an analogue of such a gibberellin), and one that ismetabolized so rapidly that no unwanted morphogenic side effects, suchas lengthening of the plastochron, are produced. Especially suitablegibberellins are those which are hydroxylated at positions in thegibberellane skeleton other than at positions 1 and 2 and/or are1,2-dihydro. Examples of such gibberellins include gibberellin A₄ (GA₄)and its salts and esters, the C-3 epimer of GA₄ and its salts and estersand gibberellin GA₁,and its salts and esters. Examples of suitable saltsand esters include the sodium salt and C₁₋₄ carboxylic acid esters.

The gibberellins used in accordance with the present invention may beapplied with other plant growth regulators, i.e. chemical thinningagents and growth retardants, but gibberellins with persistentbiological activity, especially gibberellins A₃ and A₇ should not beused. Thus, the gibberellins used in accordance with the inventionshould be substantially free of those gibberellins with persistentbiological activity.

Although the method of the invention may be carried out on trees whichare not flowering or have not flowered (e.g. to promote flowering ofjuvenile trees), it would normally be carried out during a season inwhich flowering and fruiting has already taken place, so as to stimulate`return flowering`. The intent is to counteract or over-ride anyinhibitory effect caused by the current season's flowering and/orcropping.

The application of gibberellin in accordance with the invention isdesirably carried out in late spring/early summer and, depending on thecrop, probably not later than about 12 weeks after anthesis. Goodresults have been obtained with application of gibberellin in the periodfrom 2 to 8 weeks after anthesis on apple. Although multipleapplications of gibberellin may be made, significantly improvedflowering has been achieved with a single application.

The method of application of gibberellin is not thought to beparticularly critical and may be accomplished by spraying thegibberellin to whole trees together with a suitable carrier. Theaddition of conventional adjuvants such as wetting agents anddispersants may prove to be beneficial in some agronomic situations.

Only small quantities of gibberellin need be applied in order tostimulate return flowering in accordance with the invention.Satisfactory results have been achieved with as little as 3 μg per spurand it is expected that solutions containing as little as 30 ppm of thegibberellin will give satisfactory results when applied as foliarsprays. Normally the method of invention involves the use of agibberellin as the sole plant growth modifying agent but the addition ofa natural cytokinin, zeatin, appeared to enhance the flower promotingactivity in one experiment (even though previous tests by us of this andother cytokinins alone had not been encouraging). Thus, the methodcannot rule out the possibility that the addition of other plant growthregulators might enhance activity.

Compositions for use in accordance with the method of the inventioncomprising a rapidly metabolizable gibberellin, together with ahorticulturally acceptable diluent or carrier, said compositions beingsubstantially free of gibberellins with persistent biological activity,are novel and form a further aspect of the invention.

The invention described here has great potential for improving thecropping behaviour of fruit trees prone to biennial flowering and itssuccess is particularly surprising since the weight of experimentalevidence would predict the opposite. The method of the invention asapplied to apple trees will now be described in the following examples.

EXAMPLE ONE Plant material and cultural conditions

Nine mature (21-year-old) spur-type Golden Delicious apple trees with ahistory of severe biennial bearing were selected during the 1982 blossomperiod from a larger population of trees in Orchard 1C of theSummerland, British Columbia, Canada Research Station. Each tree wasjudged to be `on` in 1982, with virtually every spur flowering. Thus,return flowering in 1983 was expected to be very low.

To improve the chances of return flowering each tree was carefully handthinned within 2 weeks of full bloom. Eighty percent of the floweringspurs were defruited and a single fruit retained on the remaining spurs.These trees received normal cultural attention during the 1982 growingseason and the fruit was harvested in mid-October. Care was taken duringthe harvest and pruning operations during dormancy to avoid removing ordamaging treated spurs and branches.

Growth regulator applications

The following treatment solutions were applied with a small paint brushto 2 fully expanded rosette leaves on each of 25 defruited spursrandomly located throughout each of the 9 trees;

(a) 60% ethanol control;

(b) 2 ppm GA₄ * in 60% ethanol;

(c) 20 ppm GA₄ * in 60% ethanol;

(d) 200 ppm GA₄ * in 60% ethanol;

(e) 20 ppm GA₄ * plus 20 ppm zeatin in 60% ethanol

Each of these treatments was applied during the period of June 14-17,1982, or about 4.5 weeks after full bloom. Treatments (b) to (e) wereapplied to additional spurs on the same trees on June 30, or about 7weeks after full bloom.

Each leaf was `painted` on all surfaces, applying about 1.5 ml ofsolution to each spur. Thus, the amount of GA4 applied ranged from 0 toapproximately 300 μg per spur. Treatment (e) applied 30 ug of GA₄ and 30ug of zeatin.

Measurements and statistical analysis

Early in the bloom period of 1983 each spur was examined for thepresence or absence of flowers. There were no apparent differences inthe number or `quality` of individual flowers per cluster so no detailedmeasurements of this nature were made. The flowering results (expressedas a percentage of flowering spurs per treatment per tree) weresubjected to an analysis of variance of 9 treatments and 9 blocks(trees).

The 200 ppm GA₄ treatment, appled at either 4.5 or 7 weeks afteranthesis in 1982, significantly increased the proportion of spurs thatproduced flowers in 1983 (Table 1). Furthermore, flowering was increasedby 3 of the 4 treatments involving 20 ppm GA₄, and the GA₄ plus zeatinmixture appeared to be superior to GA₄ alone. The 2 ppm treatmentapplied 7 weeks after bloom was ineffective and the relatively highvalue resulting from the comparable 4.5 week treatment, althoughstatistically significant, may be anomalous.

The amount of return flowering obtained with the 200 ppm GA₄ treatmentswould be enough to insure a commercial crop of apples given reasonableconditions for fruit set and development. This treatment would,therefore, be of considerable commercial interest.

However, of particular biological significance is that, contrary toprevious results with exogenously applied gibberellins A₃, A₇ and A₄plus A₇ mixtures, gibberellin A₄ significantly promoted floweringrelative to an appropriate control treatment.

                  TABLE 1                                                         ______________________________________                                        The effect (as a % of flowering spurs) of GA.sub.4 and GA.sub.4 plus          zeatin treatments (applied to defruited Golden Deliclous spurs                in 1982) on return flowering of those spurs in 1983.                                            Treatment time*                                             Treatment Concentration (mg 1.sup.-1)                                                             4.5 weeks 7 weeks                                         ______________________________________                                        Control       0         2.27.sup.a **                                         GA.sub.4      2         7.74.sup.b                                                                              2.37.sup.a                                  GA.sub.4     20         6.21.sup.b                                                                              3.95.sup.ab                                 GA.sub.4 + Zeatin                                                                          20         8.22.sup.b                                                                              6.82.sup.b                                  GA.sub.4     200        12.67.sup.c                                                                             13.71.sup.c                                 ______________________________________                                         *Weeks after full bloom in 1982.                                              **Mean Values followed by differing letter are statistically significant      at P ≦ 0.05 using Duncan's Multiple Range Test (S.E. of the mean =     1.37).                                                                   

EXAMPLE TWO Plant material and cultural conditions

Five 16-year-old Golden Delicious apple trees exhibiting severe biennialbearing were selected from a larger population of trees in Orchard 1C ofthe Summerland, British Columbia Research Station. 1978 was the `on`year for these trees and to improve the chances of obtaining a crop inthe following year they were severely hand-thinned at 20 days after fullbloom. On average, 19 of each 20 flowering spurs were defruited at thattime.

Growth regulator applications

At 22 days after full bloom 60 defruited spurs were selected and taggedon each tree, and one of the following treatments was applied to 20spurs per tree:

(a) 60% ethanol control;

(b) 25 ug of C-3-epi-GA₄ in 60% ethanol; and

(c) 50 ug of C-3-epi-GA₄ in 60% ethanol.

Each treatment was applied in approximately 250 μl of solution appliedas a fine aerosol spray to the spur rosette leaves and bourse shoot.Adjacent spurs were protected from the treatment.

Measurements and statistical analysis

Each spur that could be located (a few spurs were lost during the1978-79 dormant period) was examined in early May of 1979 for thepresence of absence of blossoms. The number of flowers in each blossomcluster was also recorded. The results were expressed as the percentageof treated spurs exhibiting flowers on each tree, and these values wereanalyzed with an analysis of variance of 3 treatments and 5 blocks.

Spurs treated with C-3-epi-GA₄ were significantly more likely todifferentiate flowers than the ethanol-treated control (Table 2).Furthermore, the response was related to the concentration of C-3epi-GA₄ applied; 50 ug per spur being significantly more effective thanthe 25 ug C-3 epi-GA₄ treatment. There was no significant treatmenteffect on the number of flowers per flowering cluster, with the averagevalues for treatments (a) to (c) being 5.1, 5.24 and 5.22, respectively.

                  TABLE 2                                                         ______________________________________                                        The effect of C-3 epi-GA.sub.4 (applied to defruited Golden Deliclous         apple spurs 22 days after full bloom in 1978) on the percentage of            spurs flowering in 1979.                                                                               Return                                               Treatment                flowering (%)                                        ______________________________________                                        (a) Control (250 μl of 60% ethanol)                                                                 29.6.sup.a *                                         (b) 25 ug C-3 epi-GA.sub.4 in 250 ul of 60% ethanol                                                    35.8.sup.b                                           (c) 50 ug C-3 epi-GA.sub.4 in 250 ul of 60% ethanol                                                    52.8.sup.c                                           ______________________________________                                         *Mean values followed by different letters differ significantly at P          ≦ 0.05. (S.E. of the mean = 4.63).                                

The significance of these findings is that even where the tendency ofthe trees was to initiate a rather high number of flowers (nearly 30% ofthe control spurs exhibited flowers compared to about 2% in Example 1) asignificant promotive effect of the gibberellin treatment was apparent.Furthermore, the method of application differed from Example 1 in thatmore of the spur leaves were treated and the material was applied in alow volume aerosol spray.

EXAMPLE THREE

Experiments as described in Examples 1 and 2 were carried out to assessthe effects on return flowering of GA₄ and GA₁ C-13-acetate applied attwo concentrations. Statistical comparisons were made between trees withhigh and low return flowering in 1985 and between spurs which were withand without fruit in 1984. The results are shown in the following Tables3 to 5.

                  TABLE 3                                                         ______________________________________                                        Percentage of flowering on spurs with fruit in 1984. Trees with               low return flowering in 1985.                                                              Tree No.                                                         Treatment      29-3   29-4   29-5 29-21                                                                              29-23                                                                              .sup.--X.sub.5                    ______________________________________                                        No spray       0.0    0.0    0.0  4.2  0.0  0.8 b                             5 ppm GA.sub.4 7.3    0.0    0.0  2.9  3.3  2.7 b                             50 ppm GA.sub.4                                                                              3.8    7.3    0.0  4.5  2.8  3.7 b                             5 ppm GA.sub.1 C-13-acetate                                                                  2.0    3.3    0.0  2.5  0.0  1.6 b                             50 ppm GA.sub.1 C-13 acetate                                                                 13.3   9.4    6.3  6.9  2.2  7.6 a                             ______________________________________                                         a, b = treatments with differing letters are statistically significantly      different at P ≦ 0.05 by Duncan's Multiple Range test             

                                      TABLE 4                                     __________________________________________________________________________    Percentage of flowering in 1985 on spurs with fruit in 1984. Trees with       high and                                                                      low return flowering combined to calculate the .sup.-- X.sub.10.                           Tree No.                                                         Treatment    29-1                                                                             29-2                                                                             29-7                                                                             29-8                                                                             29-22                                                                            29-3                                                                             29-4                                                                             29-5                                                                             29-21                                                                            29-23                                                                            .sup.--X.sub.10                    __________________________________________________________________________    No spray     0.0                                                                              7.7                                                                              28.6                                                                             4.2                                                                              13.6                                                                             0.0                                                                              0.0                                                                              0.0                                                                              4.2                                                                              0.0                                                                               5.8b                              5 ppm GA.sub.4                                                                             13.0                                                                             7.1                                                                              12.0                                                                             25.0                                                                             26.1                                                                             7.3                                                                              0.0                                                                              0.0                                                                              2.9:                                                                             3.3                                                                               9.7ab                             50 ppm GA.sub.4                                                                            16.7                                                                             0.0                                                                              12.1                                                                             28.6                                                                             22.2                                                                             3.8                                                                              7.3                                                                              0.0                                                                              4.5                                                                              2.8                                                                               9.8ab                             5 ppm GA.sub.1 C-13-acetate                                                                14.3                                                                             17.8                                                                             43.6                                                                             30.4                                                                             20.0                                                                             2.0                                                                              3.3                                                                              0.0                                                                              2.5                                                                              0.0                                                                              13.4a                              50 ppm GA.sub.1 C-13-acetate                                                               20.8                                                                             10.0                                                                             15.8                                                                             29.4                                                                             48.1                                                                             13.3                                                                             9.4                                                                              6.3                                                                              6.9                                                                              2.2                                                                              16.2a                              __________________________________________________________________________     a, b = treatments with differing letters are statistically significantly      different at P ≦ 0.05 by Duncan's Multiple Range test             

                  TABLE 5                                                         ______________________________________                                        Percentage of flowering (spurs with fruit in 1984 combined with               spurs that had no fruit in 1984) for trees with low return                    flowering in 1985.                                                                    Tree No.                                                              Treatment 29-3   29-4     29-5 29-21  29-23                                                                              .sup.--X.sub.5                     ______________________________________                                        No spray  2.5    0.0      0.5  2.1    0.5  1.2c                               5 ppm GA.sub.4                                                                          4.8    1.6      3.2  5.0    3.3  3.6b                               50 ppm GA.sub.4                                                                         2.6    4.4      1.3  2.6    1.9  2.6bc                              5 ppm GA.sub.1                                                                          2.6    3.2      1.4  2.7    0.8  2.1bc                              C-13-acetate                                                                  50 ppm GA.sub.1                                                                         7.8    6.8      8.3  4.7    1.4  5.8a                               C-13 acetate                                                                  ______________________________________                                         a, b, c = treatments with differing letters are statistically                 significantly different at P ≦ 0.05 by Duncan's Multiple Range tes

We claim:
 1. A method of promoting flowering of an apple tree which hasdemonstrated periodically inconsistent flowering, said methodcomprising:applying to said apple tree, as a foliar treatment during thespring or summer preceding the year in which increased flowering isdesired, not more than twelve weeks after flowering, a compositionconsisting essentially of a flowering-inducing effective amount of agibberellin selected from the group consisting of gibberellin A₁,gibberellin A₄, C-3 epi-gibberellin A₄ and a salt or ester thereof, saidgibberellin being applied in the substantial absence of gibberellins A₃and A₇, said effective amount being in the range of from 3 to 300micrograms of said gibberellin per spur or short shoot of said tree. 2.A method according to claim 1 wherein the ester is a C₁₋₄ carboxylicacid ester.
 3. A method according to claim 1 wherein the gibberellin isapplied from 2 to 8 weeks after full bloom.